A radio frequency (RF) signal includes useful information that is modulated onto a carrier signal. An RF receiver is a circuit configurable to retrieve the useful information from the RF signal. RF receivers are used in a wide range of applications, including television (TV) receivers (including cable and satellite set-top boxes), cellular telephones, pagers, global positioning systems, cable modems, cordless telephones, satellite radios, and other electronic devices. As used herein, the term “RF signal” refers to an electromagnetic signal having a frequency in a spectrum from about 3 kilohertz (kHz) to hundreds of gigahertz (GHz), regardless of the transmission medium through which such a signal is conveyed. Thus, an RF signal may be transmitted through air, free space, coaxial cabling, twisted pair cabling, fiber optic cabling, etc.
In many broadcast RF transmission systems, the frequency spectrum is relatively wide and is divided into separate channels (or frequency bands) that include different information. For example, a TV signal can be composed of one or multiple carriers, which can be analog modulated (ATV), or digitally-modulated (DTV), and which generally occupy a bandwidth of 6, 7 or 8 megahertz (MHZ). A TV receiver can receive such a wide spectrum RF signal, mix a desired channel within the RF signal to a convenient intermediate frequency (IF) to produce an IF signal that is easier to filter, and then convert the IF signal to a baseband signal for further processing. For example, a TV receiver may translate a selected channel having a bandwidth of approximately 6-8 MHz and a center frequency within a frequency range of approximately 48 MHz to 870 MHz to an intermediate center frequency of 44 MHz, filter the 44 MHz IF signal, and then convert the 44 MHz IF signal to a baseband frequency for further processing, such as by a digital signal processor.
Often, the power level of the RF signal in a particular channel is low, and the RF receiver amplifies the RF signal before and after mixing or otherwise processing the RF signal, which is either limited in noise or linearity performance. Thus, receivers, such as TV receivers, commonly use a technique known as automatic gain control (AGC) to amplify the RF signal to an appropriate power level.
High quality television receivers have traditionally included AGC circuitry that adjusts the gain or attenuation of various elements in the receiver, regulating power levels within the receiver circuitry. For example, a television signal with low input power can be amplified to increase the signal strength for further processing. In another example, a filtered signal may be too powerful for another component in the signal path, and so the filtered signal can be attenuated to decrease the power level. Without such AGC circuitry, the displayed image of an analog television signal would become dimmer when the power level decreases and brighter as the power level increases.
In receivers with AGC circuitry, sudden changes in the input power level can cause undesirable operation, causing the displayed image to appear to flicker and the sound track to include noticeable changes in volume or even lose video or audio reception for brief periods. The levels of the signals can change suddenly when, for example, a moving receiver passes into a tunnel or behind a building, or an obstruction, such as an airplane, passes between the transmitter and the receiver.
To efficiently implement AGC in highly integrated receivers, the gain or attenuation of the various elements can be controlled discretely in small gain steps. However, to achieve the desired gain through such small gain steps, a large number of small gain or attenuation elements are switched on or off in order to achieve the desired gain. Such small gain or attenuation elements are easily created in an integrated circuit process technology, such as a complementary metal-oxide semiconductor (CMOS) process. However, the large number of gain or attenuation elements leads to larger die sizes and higher production costs. Moreover, for a given technology there may be a physical limit to what level of resolution is achievable, and requirements for good reception of video signals approach that limit.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items.